This section provides background information related to the present disclosure that is not necessarily prior art.
A pump which has two pump sections with each pump section having one suction region and one pressure region is apparent from European Patent Application EP 0 758 716 A2. A pressure space with an outflow region to a consumer is provided, the pump delivering, during its operation, a fluid from the suction regions into the pressure space and via the outflow region further to a consumer. The pump has a rotor which is operatively connected to a shaft which can be rotated about a rotational axis. Delivery elements are received displaceably, as viewed in the radial direction, in the rotor, the delivery elements being configured as vanes, with the result that the known pump overall is configured as a vane cell pump. The function of the pump is configured in such a way that the rotor rotates within a contour ring during operation, driven by the shaft, two crescent-shaped delivery spaces being formed by it which are passed through by the delivery elements which can be displaced in the radial direction. This results during the rotation of the rotor in spaces which become larger and smaller, namely the suction and pressure regions. Radially within the delivery elements, the rotor has expelling regions which are connected at least partially to at least one pressure region via a first fluid path. In the vane cell pump, for example, lower vane grooves are provided by way of which the expelling regions are fluidically connected to at least one pressure region, in order to expel the vanes during starting of the pump. During operation of the pump, the delivery elements are driven radially to the outside not only by way of the centrifugal forces which act on account of the rotor rotation, but also assisted by the pump pressure which prevails in the expelling regions via the first fluid path, with the result that they run sealingly on an inner circumferential face of the contour ring. The pump is typically arranged in such a way that its rotational axis extends substantially in the horizontal direction. If the pump is brought to a standstill in a warm operational state, the delivery elements which lie at the top slide into their receptacles which are provided on the rotor on account of gravity, as a result of which the separation which otherwise exists between the suction and pressure region as a result of the delivery elements is dispensed with. This produces as it were a short-circuit in the pump section which lies at the top. The delivery elements which lie at the bottom remain in contact with the contour ring as a result of gravity, with the result that the suction and the delivery region are separated here by way of the extended delivery elements.
If the fluid which is delivered by the pump, for example a hydraulic oil, now gets colder, its viscosity increases with the result that the mobility of the delivery elements decreases. If the pump is started up, at any rate a greatly reduced delivery capacity is produced in the case of cold starting on account of the short-circuit in one pump section. In order to avoid this problem, a cold starting device is provided in the pump disclosed in European Patent Application EP 0 758 716 A2, which cold starting device comprises a cold starting element which is prestressed into a first functional position, in the form of a cold starting plate. In its first functional position, the cold starting element shuts off a second fluid path which leads from the pressure regions to the pressure space. At the same time, a fluidic connection between the two pressure regions of the two pump sections is preferably also shut off by way of the cold starting element. In a second functional position, the cold starting element releases the second fluid path. Here, the cold starting element is configured and arranged in such a way that it can be displaced counter to the prestress into its second functional position by way of a pump pressure which is generated in the pressure regions during operation of the pump. In the first functional position, there is no fluidic connection between the pressure regions and the pressure space with the result that the fluid which is delivered by the pump when running up is delivered completely via the first fluid path into the expelling regions. In this way, the delivery elements are displaced out of their receptacles in the rotor with the result that the short-circuit which exists at a standstill between the suction and the pressure region is closed. The first fluid path is particularly preferably configured in such a way that it supplies expelling regions of this type with fluid, which expelling regions are just moving through a suction region, as viewed in relation to the rotation of the rotor. Therefore, during cold starting, the pump rapidly reaches its complete delivery capacity. If the pump pressure in the pressure regions exceeds the prestressing force which holds the cold starting element in its first functional position, the cold starting element is displaced counter to the prestress into its second functional position with the result that it then also releases the second fluid path which leads from the pressure regions to the pressure space. Therefore, in the case of sufficient pump pressure, fluid is then also delivered through the pressure space and via the outflow region to the consumer.
It is disadvantageous here that, during the operation of the pump, the cold starting element is constantly loaded on a side which faces away from the pressure regions with a system pressure which prevails in the outflow region. Two force components which are added together therefore act on the cold starting element in the direction of the first functional position, namely firstly the prestressing force and secondly the force which acts on account of the system pressure which prevails in the outflow region. These forces have to be balanced by the pump pressure during operation of the pump in order to hold the cold starting element permanently in its second functional position. The pump pressure in the at least one pressure region therefore always has to be greater by an amount which corresponds to the prestressing force than the system pressure which prevails in the outflow region. This additional pressure difference has to be applied permanently by the pump, as a result of which said pump has an increased power consumption.